1. Field of the Invention
This invention relates to a zoom lens system, and is suitable, for example, as the photo-taking optical system of a photographic camera, a video camera, a digital still camera or the like.
2. Description of the Related Art
When a shake is accidentally transmitted to a photographing system, blur occurs to a photographed image. There have heretofore been proposed various zoom lenses provided with a function of compensating for the blur of an image (image stabilizing function) due to such accidental shake. There are known, for example, optical systems which compensate for image blur due to a shake with a part of a lens unit constituting the optical system (zoom lens) moved in a direction substantially perpendicular to the optical axis thereof (Japanese Patent Application Laid-open No. H08-136862 (corresponding to U.S. Pat. No. 6,124,972), Japanese Patent Application Laid-open No. H07-325272, Japanese Patent Application Laid-open No. H09-230237 (corresponding to U.S. Pat. No. 6,266,189), Japanese Patent Application Laid-open No. H10-039210 (corresponding to U.S. Pat. No. 5,835,272), Japanese Patent Application Laid-open No. H11-231220 and Japanese Patent Application Laid-open No. H10-090601 (corresponding to U.S. Pat. No. 6,025,962).
Generally, when a photographing system is inclined by a shake, a photographed image is displaced by an amount conforming to the angle of inclination thereof and the focal length of the photographing system. Therefore, in an image pickup apparatus for a still image, there is the problem that a photographing time must be made sufficiently short to prevent the quality of image from deteriorating, and in an image pickup apparatus for a moving image, there is the problem that it becomes difficult to maintain the setting of composition. Therefore, in case of such photographing, it becomes necessary to correct so that the displacement of a photographed image, i.e., the so-called blur of a photographed image, may not occur even when the photographing system is inclined by a shake. Japanese Patent Application Laid-open No. H08-136862 discloses an embodiment suitable for being applied chiefly to a standard zoom lens for a single-lens reflex camera. Japanese Patent Application Laid-open No. H08-136862 discloses a construction which compensates for the blur of a photographed image by moving a second lens unit in a direction substantially perpendicular to the optical axis thereof, in a four-unit zoom lens comprised, in succession from the object side, of a first lens unit having positive refractive power, the second lens unit having negative refractive power, a third lens unit having positive refractive power and a fourth lens unit having positive refractive power.
Also, Japanese Patent Application Laid-open No. H07-325272, Japanese Patent Application Laid-open No. H09-230237, Japanese Patent Application Laid-open No. H10-039210 and Japanese Patent Application Laid-open No. H11-231220 disclose a form in which a movable lens unit is divided into two lens components to make shake correction performance and other performance compatible, and one lens component is a lens unit for shake correction.
Japanese Patent Application Laid-open No. H10-90601 discloses a construction which compensates for the blur of an image by moving the fourth lens unit having negative refractive power of a five-unit zoom lens comprised, in succession from the object side to the image side, of a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having positive refractive power, a fourth lens unit having negative refractive power and a fifth lens unit having positive refractive power.
In recent years, in single-lens reflex cameras, a digital single-lens reflex camera using a solid-state image pickup device such as a CCD sensor or a CMOS sensor as an image sensor is becoming a mainstream instead of a conventional silver-halide film camera. As this digital camera, there is demanded one of a simple construction.
Also, there have been manufactured a number of digital single-lens reflex camera using an image sensor of a size similar to APS size smaller than the same 135 size as silver-halide film, and a photo-taking optical system suitable therefor is also demanded. When the size of the image sensor becomes small, the field angle becomes narrow as compared with the 135 size and therefore, if in a zoom lens covering a wide angle to medium telephoto focal length, a zoom lens in which the image size is the 135 size is intactly made to have the same field angle, the entire optical system becomes bulky. For example, if in a zoom lens used in a camera of APS size, an attempt is made to obtain the same field angle as that of a zoom lens having a focal length of 28-135 mm and directed to the 135 size, the focal length will become 17.5-85 mm. If a zoom lens having a focal length of 17.5-85 mm is designed at an image size of the 135 size, the optical system will become bulky.
So, there is demanded an exclusive zoom lens corresponding to an image size smaller than the 135 size, for example, APS size.
On the other hand, of a mechanism which shakes some lens units of a photographing system to thereby eliminate the blur of an image and obtain a still image, it is required that the correction amount of the blur of the image be great, that the amount of movement or the amount of rotation of a lens unit to be shaken in order to correct the blur of the image (image stabilizing lens unit) be small, that the entire apparatus be compact, and the like.
Now, assuming that the ratio |Δx/Δh| of the correction amount Δx of the blur of an image to the unit movement amount Δh of the image stabilizing lens unit in a direction perpendicular to the optical axis thereof is defined as eccentricity sensitivity TS, the eccentricity sensitivity TS is defined asTS=|Δx/Δh|. Also, letting f be the focal length, and BS (degrees/mm) be shake correction sensitivity which is defined asBS=(180/π)×TS/f, the shake correction sensitivity BS is indicative of a shake correction angle relative to the unit movement amount of the image stabilizing lens unit in the direction perpendicular to the optical axis thereof. The accuracy with which the image stabilizing lens unit is controlled depends greatly on the resolution of an actuator which moves the image stabilizing lens unit, and when the shake correction sensitivity BS is too high, a problem occurs to stopping accuracy. Also, when the shake correction sensitivity BS is too low, the amount of movement of the image stabilizing lens unit for the purpose of shake correction becomes great, thus resulting in an increase in electric power consumption and the bulkiness of the optical system.
FIG. 18 of the accompanying drawings is an illustration showing the field angle characteristic of the shake correction sensitivity BS.
Generally, as shown in FIG. 18, it is necessary for the shake correction sensitivity BS be within a desired value range in conformity with the field angle of the optical system.
Further, in the case of an optical systems of the same field angle, it is preferable that the shake correction sensitivity BS be of a value within the same range irrespective of the size of an image circle.
In a zoom lens provided with a image stabilizing function directed to an image pickup device of an image circle smaller than the 135 size, if an attempt is made to design an optical system of the same field angle as the 135 size, there is the problem that the eccentricity sensitivity BS of the image stabilizing lens unit at the telephoto end becomes too high and highly accurate shake correction performance becomes unobtainable. If an attempt is made to intactly apply an optical system for the 135 size to an optical system of an image circle of a size smaller than the 135 size, for example, APS size, shake correction sensitivity becomes high in inverse proportion to the difference in the focal length. That is, assuming that in an optical system of the 135 size and having a focal length of 28-135 mm, the shake correction sensitivity BS at the telephoto end is 0.7 degree/mm, the focal length is 17.5-85 mm for APS size and the shake correction sensitivity at the telephoto end is 0.7×135÷85=1.1 degree/mm. The field angle is the same and therefore, even in the case of APS size, it is necessary to provide the same degree of shake correction sensitivity BS, but if an attempt is made to make the eccentricity sensitivity TS low, it is necessary to make the optical power (inverse number of the focal length) of the image stabilizing lens unit small, and this results in the aggravation of aberrations and an increase in the entire length of the optical system.
A construction in which a movable lens unit of negative refractive power is divided into two lens components having negative refractive power is disclosed in a first embodiment and a second embodiment of the aforementioned Japanese Patent Application Laid-open No. H07-325272, a first embodiment and a fourth embodiment of Japanese Patent Application Laid-open No. H09-230237, a first embodiment of Japanese Patent Application Laid-open No. H10-039210 and first to seventh embodiments of Japanese Patent Application Laid-open No. H11-231220.
In the first embodiment and the second embodiment of Japanese Patent Application Laid-open No. H07-325272, disclosed is a zoom lens comprised of a plurality of lens units including a first lens unit having positive refractive power and in which a lens unit having negative refractive power is constituted by two lens components having negative refractive power and shake correction is effected by one of the lens components, and this lens unit having negative refractive power is a second lens unit disposed on the object side with respect to an aperture stop. Also, the eccentricity sensitivity TS of the lens component effecting shake correction is −1.615 in the first embodiment, and −17.43 in the second embodiment.
Also in the first embodiment and the fourth embodiment of Japanese Patent Application Laid-open No. H09-230237, disclosed is a zoom lens constituted by a plurality of lens units including a first lens unit having positive refractive power, and in which a lens unit having negative refractive power is constituted by two lens components having negative refractive power and shake correction is effected by one on the lens components, and this lens unit having negative refractive power is also disposed on the object side with respect to an aperture stop. Also, the shake correction sensitivity of the lens component effecting shake correction is −1.644 in the first embodiment, and −1.650 in the fourth embodiment.
In the first embodiment of Japanese Patent Application Laid-open No. H10-039210, disclosed is a zoom lens in which a lens unit having negative refractive power is constituted by two lens components having negative refractive power and shake correction is effected by one of the lens components, and the eccentricity sensitivity TS of the lens component effecting shake correction is 1.631.
In the first to seventh embodiments of Japanese Patent Application Laid-open No. H11-231220, disclosed is a zoom lens in which a lens unit having negative refractive power is constituted by two lens components having negative refractive power and shake correction is effected by one of the lens components, and the eccentricity sensitivity TS of the lens component effecting shake correction is 1.692, 1.553, 1.551, 1.716, 1.691, 1.687 and 1.623 in the respective embodiments.
As described above, in these conventional examples, disclosed is a zoom lens in which the lens unit having negative refractive power is divided into two lens components and shake correction is effected by one of the lens components, but in any one of these conventional examples, the eccentricity sensitivity of the image stabilizing lens component is 1.5 or greater, and this is considerably great. This has led to the problem that it is very difficult to manufacture each lens unit.